This invention relates to the formulation and manufacture of a spray dry resole type phenol-aldehyde resin containing water soluble oxo compounds of boron. The powdered resin has pre-cure resistant property and finds particular utility for making composition boards such as that of waferboard and oriented strandboard (OSB) manufacture.
At present, most Canadian waferboards and oriented strandboards are manufactured with powdered phenol-formaldehyde resins. The powdered resins have a number of advantages over liquid resins, such as: (a) simple and less expensive equipment can be used for powder resin delivery and blending with wood furnish; (b) powdered resins are easy to blend onto wood wafers and strands, achieving uniform resin distribution; and (c) powdered resins have long storage lives.
In industrial practice, resins with different properties are applied separately onto face and core furnish of waferboard/oriented strandboard. The face resin blended wood furnish is used for face layers, and the cored resin blended wood furnish is for core layer construction of waferboard/oriented strandboard. The face resin requires pre-cure resistance, non-sticking to platen and light color after cure. The core resin requires the ability to cure fast. The two resin (face and core) system has the advantages of producing high quality board and increasing mill productivity by reducing hot pressing times.
In earlier industrial practice, novolac type phenol-formaldehyde powder resins were used for waferboard/oriented strandboard manufacture. The novolac resins have excellent pre-cure resistant properties due to excellent thermal flow under heat and pressure, and they also have a light color on the wood board surface after curling. However, the novolac resins have the disadvantages of sticking on caul plate and platen surfaces and being of considerably higher cost to manufacture than spray dried resole resins.
For waferboard/oriented strandboard manufacture, the spray dried resole resins require a property of pre-cure resistance for face layer applications. To have the pre-cure property, the powder resin must have a significantly longer thermal flow property under heat and pressure than that of the core resin. The thermal flow property is mainly controlled by the molecular weight of the resin. Low molecular weight resins show higher thermal flow property and better pre-cure resistance than high molecular weight resin. However, spray drying a resole resin having the desired pre-cure resistance is difficult due to poor spray dryability of the low moleuclar weight resin.
In the spray drying process, a liquid phenol-aldehyde resin is atomized to fine droplets and mixed with hot air (180.degree. to 210.degree. C.) to evaporate the water from the resin droplets. The temperature of the resin is usually close to the boiling water temperature or higher. After evaporation of water, the low molecular weight phenol-aldehyde resin particles are soft and tacky. The low softening particles tend to deposit on the surfaces of the spray dryer chamber and cyclone. The undesired deposit of resin is severely detrimental to the spray drying operation by reducing powder resin yield, increasing fire hazard, frequently shutting down operation for cleaning, and undesirably increasing powder particle size. Therefore, the spray drying of low molecular weight resole has the problems of low powder resin yield, high cost of production, low spray dryer productivity, and the production of inferior products.